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FAQs 

Electrical

Strings
The cartoon above describes how panels are clipped together in 'series'..which simply means a positive to negative connection of the MC4 plugs on the back of every panel.

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A typical 275W panels produces 35 volts at the moment the sun hits it and before there is much load on it, and then settles down to around 25V for the rest of the day. How much current it produces is down to the strength of the irradiation from the sun. At 9am there might be a single Ampere of current, so 25 x 1 = 25W of power from your theoretically possible 275W, and at mid day 8A, so 25 x 8 = 200W of power, but in real life conditions you will NEVER get the full 275W. That's a factory lab maximum only under perfect temp and light conditions.

The panels are strung together in series, as pictured, so that each panel on the string contributes to increased voltage but the current is uniform. And therein lies one of solar's greatest problems. If a panel is shaded then it's voltage will largely stay the same but its current can drop dramatically. You can't have a string of panels where one panel is at one lower current and the rest at a higher one, so the current for the entire string drops to the lowest current. Very bad news as far as power production is concerned.

That's why, as solar designers, we design a system where none of the panels get any shade even if it means you get less panels or we provide power optimisers that can be selectively deployed on panels that will be shaded to negate the effect on other panels. See the first FAQ on this section for how that works !!

Parallel strings

A string of panels usually can not exceed 14 panels. Even though most times the total string voltage from 14 panels will be less than 400V and inverters can usually handle at least 500V there are times, cold days, early mornings, when the voltage can jump up. It was common in the "old days" of solar, you know, pre-2013, to parallel strings together so we could double the current but leave the voltage as it was. That's not possible with almost all modern inverters. Panels these days produce a higher current and almost all inverters aren't designed for doubling up on that higher current.

Clipping
If you look at the voltage and current produced by a string of panels controlled by an inverter MPPT, you'll see that the voltage produced by the panels gets up to their optimal level (VMP) very quickly and stays that way all day unless severe amounts of clouds pass by, but the current produced is a bell curve, gradually rising to a peak, typically at mid day on a North facing roof, and falling away as the power of the sun declines. If your installation has been designed so that the inverter maximum current limit has been exceeded (e.g. a parallel of two strings with 18 amps on a 16 amp limit), then at mid day, it's possible that the inverter will clip off the excess current, resulting in you losing some power production (Volts x Amps = Watts). The same applies if the designer has exceeded the maximum input voltage of the inverter. It won't harm the inverter, it just means that the excess is clipped off. For those who go down this path, the overall power production against losses from clipping is overwhelmingly in their favour.

How many panels can be connected to an inverter MPPT?

It does depend on the panel, but for a regular panel 265W to 300W, 14 is a safe number.

Start up voltage
An inverter MPPT needs more juice to start up, than it does to stay working.

 

 

 




 

 

 

 

 

 


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